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noise to signal ratio

Ive tuned up a bunch of radios in the past so i know enough to get myself in trouble. Hi. But...
This signal to noise ratio i just cant seem to get. When tuning the rx i use an old volt meter from the 70's and plug it into the external speaker. When tuning i keep turning (the correct pots) and watch the meter climb. I know im done when the meter stops climbing.

What is a better way of doing this? What piece of equipment should i be looking to buy to get the noise to signal ratio better?

Staff Member

When ever I have tuned receivers, granted most of them were commercial FM units but the principles are the same, I used an RF signal generator on the antenna input and an audio harmonic distortion analyzer on the speaker output. A simple VTVM or other high impedance voltmeter would accomplish the same on the speaker output. I would set up a weak RF signal and tune the receiver for MINIMUM noise from the speaker. If I achieved full quieting i would reduce the RF input to the point the signal got noisy again and repeat until there was no more improvement. Then I would remove the signal and measure the noise present with no signal. Next I would reconnect the signal generator and increase the input until the noise was 10 dB below the level present with no signal input. This gave me the signal required for 10dB S/N ratio which is pretty much a standard.

I agree with Capt K as that is a proper method.
Using a SINAD meter is the preferred tool if you have the bux. If you don't want to spend a bunch, there is another way to accomplish the same end. I recently discovered this SINAD meter program online; think it is definitely a cool thingie. You can make your own cable (or buy their cable) and use your computer. The program also has a trial version so you can use it before you buy it. I still have to make the cable and will start another thread and takes some pics when I do. Only $39 (to licensed Amateurs for non-commercial use). Kewl! A LOT cheaper than buying a ~$200 SINAD meter.

Just tried out that SINAD program. It makes it sooooo much easier to tune the receiver and the receive will definitely have less signal distortion. It worked just fine. Very useful features. I haven't taken any before/after screenshots yet.

Cost me $2 for the 6' cable with male/male 1/8" plug ends at a surplus electronics store. Just plug one end into the 'line in' on the computer's sound card, the other end of the cable goes into the external speaker jack on the back of the radio, turn on and hook up your RF sig gen to the radio, and then set up the SINAD program as described in the User Manual file.

Did two radios in about 20 minutes. Used it on the SS/JAcko and a RCI-2950 that were already opened and being worked on.

Thanks for the replys guys. Ill try what CK was saying and look into the program rob is talking about.
Rob, looking at the pic you posted it shows the computer connected to the SG. Mine is so old i dont have a way to do that. Is this nessasary to use the program?

No; the output of the RF sig gen is connected to the SO-239 socket of the radio - just like you would normally do..

The external speaker output jack found on the back of your radio connects via that 6 ft cable mentioned above to the 'line in' on the computer sound card.

That diagram shows that cable as an option - or as it said 'if needed'. That third wire goes to the sig gen input from the computer's sound card output and should only be used if your sig gen cannot synthesize its own 1khz audio signal. Most modern sig gens today have that feature already; so do the better older generators.

If you want to do a receive alignment that is rock solid; then this SINAD program is the best, cheapest solution I have found. If you just use a sig gen; then it simply cannot detect the receiver's distortion. It is just a signal source and nothing more. Using just a sig gen alone will keep your radio from being as sensitive as it is capable of. Distortion and noise are a product of the radios circuitry and can be detected in the audio circuit. A scope may help; but it doesn't show the SINAD ('signal-to-noise and distortion ratio') and THD ('total harmonic distortion') as identifiably as this program does. Absolutely impossible to beat for the very small cost they ask.

I still have one of those old sig gens; but now it makes a fine doorstop. Was really grateful I found a real sweet deal on a Hp unit here in Silicon Valley; but that doesn't mean it was free nor cheap. This SINAD program is worth more than they sell it for; go find the price of a real SINAD meter and see for yourself.
Just my two cents . . .

Intrested to know if you could measure what the S/N was with the 1uV RF input at 100% modulation with that tool.

With a 1 KHz test tone I measured a little over 50 dB S/N with a 1 mV RF input, which is 30dB over threshold. Of course I measured it with the LF section of the HP 8556B, which also measured even and odd harmonics every 1 KHz from the fundamental.

The S/N was good, but the harmonics were down only -27 dB second and -25 dB third, for a THD in the 7% range. It was pretty consistent at various volume levels.

"Maximum Recovered Audio" and "Best Signal to Noise Ratio" don't occur at the same tunning adjustments at the front end.
It is best to tune for Best Signal to Noise Ratio and that is what the SINAD meter lets you do.
Remember to use the least amount of signal level from the generator that will allow the SINAD meter to function properly.
If you use too much signal from the generator you will never be able to detect the point of adjustment that provides the best signal to noise.

I think I will need to upgrade my signal generator, Maybe.
The one I have is a BK Precision solid state RF Signal Generator model number E2000. On the front of it it shows attenuator measurements in db. And on the little meter it says that zero db = 100mv. Now I don't know how to change the db to uv or whatever it is I need to be setting it too.

You can look this thing up on line and hopefully someone can help me convert the numbers. Maybe its just not going to go low enough. I did do a mod to it so that I can plug in my frequency counter into a full strength signal and fine tune it onto frequency I need give or take 0.002

Am I just out of luck and need to buy a new one or what?

Thanks for all the help in this. I know I'll get it figured out in a week or two even if I need to buy another sig gen.

OK, Now that I finally got home. Its tough driving truck and doing things at home.

I've had some time to look at the chart and I'm just not getting it. I think its just above what I know. I guess I need someone to help me to understand the ratio of DB to DBM so I can figure out what DB I need to set my sig. gen. to to do a RX alignment. Sense it only has DB settings.
OR
Just tell me the setting I need to put my sig. gen. on in DB to do this alignment.
1uv is = to ? db
and
100uv is = to ? db
It sure would be nice to get this last stage of the alignment done on my rig before I go back on the road Sat.

I will try the way that was mentioned first then DL the program that Robb showed and do it again. I figure both ways will help me to learn better.

Thanks again for all the help with this.
Jerry...

BTW, I've been going with 50db on my gen. to set S-9. Please tell me this is right or not.
Thanks

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